Semiconductor device

ABSTRACT

An object is to provide a semiconductor device having a novel structure, which can hold stored data even when not powered and which has an unlimited number of write cycles. A semiconductor device includes a memory cell including a widegap semiconductor, for example, an oxide semiconductor. The memory cell includes a writing transistor, a reading transistor, and a selecting transistor. Using a widegap semiconductor, a semiconductor device capable of sufficiently reducing the off-state current of a transistor included in a memory cell and holding data for a long time can be provided.

TECHNICAL FIELD

An embodiment of the invention disclosed herein relates to a semiconductor device including a semiconductor element and a driving method of the semiconductor device.

BACKGROUND ART

Memory devices including semiconductor elements are broadly classified into two categories: volatile memory devices that lose stored data when not powered, and nonvolatile memory devices that hold stored data even when not powered.

A typical example of volatile memory devices is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is stored in a capacitor.

When data is read from a DRAM, electric charge in a capacitor is lost according to the above-described principle; thus, another writing operation is necessary every time data is read out. Moreover, a transistor included in a memory element has leakage current (off-state current) between a source and a drain in an off state or the like and electric charge flows into or out of the transistor even if the transistor is not selected, which makes a data holding period short. For that reason, writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, another memory device utilizing a magnetic material or an optical material is needed in order to hold the data for a long time.

Another example of volatile memory devices is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation, which is an advantage over a DRAM. However, cost per storage capacity is high because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.

A typical example of nonvolatile memory devices is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding charge in the floating gate. Therefore, a flash memory has advantages in that the data holding period is extremely long (semi-permanent) and refresh operation which is necessary to volatile memory devices is not needed (e.g., see Patent Document 1).

However, in a flash memory, there is a problem in that a memory element becomes unable to function after a predetermined number of writing operations because a gate insulating layer included in the memory element deteriorates due to tunneling current generated in writing operations. In order to reduce effects of this problem, a method in which the number of writing operations is equalized among memory elements can be employed, for example, but a complex peripheral circuit is needed to realize this method. Moreover, even when such a method is employed, the fundamental problem of lifetime cannot be resolved. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.

In addition, high voltage is necessary in order to inject charge into the floating gate or removing the charge, and a circuit for that purpose is required. Further, it takes a relatively long time to inject or remove electric charge, and it is not easy to increase the speed of writing or erasing data.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.     S57-105889

DISCLOSURE OF INVENTION

In view of the foregoing problems, an object of one embodiment of the disclosed invention is to provide a semiconductor device having a novel structure, which can hold stored data even when not powered and which has an unlimited number of write cycles.

In one embodiment of the disclosed invention, a semiconductor device is formed using a material capable of sufficiently reducing the off-state current of a transistor, such as an oxide semiconductor material that is a widegap semiconductor. The use of a semiconductor material capable of sufficiently reducing the off-state current of a transistor allows data to be held for a long time.

In one embodiment of the disclosed invention, a semiconductor device has a memory cell including a widegap semiconductor, for example. The memory cell includes a writing transistor, a reading transistor, and a selecting transistor.

More specifically, the following structures can be employed, for example.

An embodiment of the present invention is a semiconductor device which includes a write word line, a read word line, a bit line, a source line, a signal line, a memory cell array having a plurality of memory cells, a first driver circuit having a delay circuit electrically connected to the signal line, and a second driver circuit. One of the memory cells includes a first transistor which includes a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region, a second transistor which includes a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region, and a third transistor which includes a third gate electrode, a third source electrode, a third drain electrode, and a third channel formation region. The first channel formation region includes a semiconductor material different from that of the second channel formation region. The first gate electrode and the second drain electrode are electrically connected to each other and form a node where electric charge is held. The first drain electrode and the third source electrode are electrically connected to each other. The source line is electrically connected to the first source electrode. The first driver circuit is electrically connected to the third drain electrode through the bit line and electrically connected to the second source electrode through the delay circuit and the signal line. The second driver circuit is electrically connected to the third gate electrode through the read word line and electrically connected to the second gate electrode through the write word line.

An embodiment of the present invention is a semiconductor device which includes a write word line supplied with at least a potential higher than a power supply potential, a read word line, a bit line, a source line, a signal line, a memory cell array having a plurality of memory cells, a first driver circuit having a delay circuit electrically connected to the signal line, a second driver circuit, and a potential conversion circuit. One of the memory cells includes a first transistor which includes a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region, a second transistor which includes a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region, and a third transistor which includes a third gate electrode, a third source electrode, a third drain electrode, and a third channel formation region. The first channel formation region includes a semiconductor material different from that of the second channel formation region. The first gate electrode and the second drain electrode are electrically connected to each other and form a node where electric charge is held. The first drain electrode and the third source electrode are electrically connected to each other. The source line is electrically connected to the first source electrode. The first driver circuit is electrically connected to the third drain electrode through the bit line and electrically connected to the second source electrode through the delay circuit and the signal line. The second driver circuit is electrically connected to the third gate electrode through the read word line and electrically connected to the second gate electrode through the write word line. The potential conversion circuit is configured to output the potential higher than the power supply potential to the second driver circuit.

Each of the above semiconductor devices may include a capacitor having one electrode electrically connected to the node where electric charge is held and the other electrode electrically connected to the source line.

In the above semiconductor device, the second driver circuit may include a step-up level shifter electrically connected to the potential conversion circuit and the write word line.

In each of the above semiconductor devices, the plurality of memory cells may be connected in parallel between the bit line and the source line.

In each of the above semiconductor devices, the second channel formation region of the second transistor may include an oxide semiconductor. The third channel formation region may include the same semiconductor material as the first channel formation region.

Note that although, in the above embodiments, the transistor may be formed using an oxide semiconductor, the disclosed invention is not limited thereto. A material capable of realizing off-state current characteristics comparable to those of an oxide semiconductor, for example, a widegap material (specifically, a semiconductor material having an energy gap Eg of more than 3 eV, for example), such as silicon carbide, or the like may be employed.

Note that the term such as “over” or “below” in this specification and the like does not necessarily mean that a component is placed “directly on” or “directly under” another component. For example, the expression “a gate electrode over a gate insulating layer” does not exclude the case where a component is placed between the gate insulating layer and the gate electrode.

In addition, the term such as “electrode” or “wiring” in this specification and the like does not limit a function of a component. For example, an “electrode” can be used as part of a “wiring”, and the “wiring” can be used as part of the “electrode”. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” is formed in an integrated manner.

Functions of a “source” and a “drain” are sometimes interchanged with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification and the like.

Note that the term “electrically connected” in this specification and the like includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an object having any electric function as long as electric signals can be transmitted and received between components that are connected through the object.

Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions as well as an electrode and a wiring.

Since the off-state current of a transistor including an oxide semiconductor is extremely small, stored data can be held for an extremely long period when the transistor is used. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be held for a long period even when power is not supplied (note that the potential is preferably fixed).

Further, a semiconductor device according to the disclosed invention does not need high voltage for data writing and does not have the problem of element deterioration. For example, unlike a conventional nonvolatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not arise. That is, the semiconductor device according to the disclosed invention has no limitation on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, since data is written by turning on or off the transistor, high-speed operation can be easily realized. Additionally, there is an advantage in that operation for erasing data is not needed.

Since a transistor including a material other than an oxide semiconductor can operate at sufficiently high speed, when this is combined with a transistor including an oxide semiconductor, a semiconductor device can perform operation (e.g., data reading) at sufficiently high speed. Further, a transistor including a material other than an oxide semiconductor can favorably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed.

A semiconductor device having a novel feature can be realized by being provided with both the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small).

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are circuit diagrams of a semiconductor device.

FIGS. 2A and 2B are circuit diagrams of a semiconductor device.

FIG. 3 is a circuit diagram of a semiconductor device.

FIG. 4 is a timing chart for a semiconductor device.

FIGS. 5A to 5E are circuit diagrams of a semiconductor device.

FIGS. 6A and 6B are circuit diagrams of a semiconductor device.

FIG. 7 is a circuit diagram of a semiconductor device.

FIG. 8 is a circuit diagram of a semiconductor device.

FIGS. 9A and 9B are circuit diagrams of a semiconductor device.

FIGS. 10A to 10C are circuit diagrams of a semiconductor device.

FIGS. 11A and 11B are a cross-sectional view and a plan view of a semiconductor device.

FIGS. 12A to 12D are cross-sectional views illustrating a manufacturing process of a semiconductor device.

FIGS. 13A to 13C are cross-sectional views illustrating a manufacturing process of a semiconductor device.

FIGS. 14A to 14C are cross-sectional views illustrating a manufacturing process of a semiconductor device.

FIGS. 15A to 15C are cross-sectional views illustrating a manufacturing process of a semiconductor device.

FIG. 16 is a plan view of a semiconductor device.

FIGS. 17A to 17F each illustrate an electronic device including a semiconductor device.

FIG. 18 is a graph showing characteristics of a transistor including an oxide semiconductor.

FIG. 19 is a circuit diagram of an element for evaluating characteristics of a transistor including an oxide semiconductor.

FIG. 20 is a timing chart for an element for evaluating characteristics of a transistor including an oxide semiconductor.

FIG. 21 is a graph showing characteristics of a transistor including an oxide semiconductor.

FIG. 22 is a graph showing characteristics of a transistor including an oxide semiconductor.

FIG. 23 is a graph showing characteristics of a transistor including an oxide semiconductor.

FIG. 24 is a cross-sectional view illustrating a structure of a transistor used for calculation.

FIG. 25 is a graph showing calculated characteristics of a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that the modes and details of the present invention can be modified in various ways without departing from the spirit and the scope thereof. Therefore, the present invention should not be interpreted as being limited to the description in the following embodiments.

Note that the position, size, range, or the like of each component illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like as disclosed in the drawings and the like.

Note that ordinal numbers such as “first”, “second”, and “third” in this specification and the like are used in order to avoid confusion among components, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a circuit configuration and an operation of a semiconductor device according to one embodiment of the disclosed invention will be described with reference to FIGS. 1A and 1B, FIGS. 2A and 2B, FIG. 3, FIG. 4, FIGS. 5A to 5E, FIGS. 6A and 6B, FIG. 7, FIG. 8, and FIGS. 9A and 9B. Note that in some circuit diagrams mentioned below, “OS” is written beside a transistor in order to indicate that the transistor includes an oxide semiconductor.

<Basic Circuit>

First, a basic circuit configuration and an operation thereof will be described with reference to FIGS. 1A and 1B. In a semiconductor device illustrated in FIG. 1A, a first wiring (1st Line) is electrically connected to a source electrode (or a drain electrode) of a transistor 160. A second wiring (2nd Line) is electrically connected to a drain electrode (or a source electrode) of a transistor 166. A third wiring (3rd Line) is electrically connected to a source electrode (or a drain electrode) of a transistor 162. A fourth wiring (4th Line) is electrically connected to a gate electrode of the transistor 162. A gate electrode of the transistor 160 is electrically connected to the drain electrode (or the source electrode) of the transistor 162. The drain electrode (or the source electrode) of the transistor 160 is electrically connected to the source electrode (or the drain electrode) of the transistor 166. A fifth wiring (5th Line) is electrically connected to a gate electrode of the transistor 166.

Here, a transistor including an oxide semiconductor is used as the transistor 162, for example. A transistor including an oxide semiconductor has a characteristic of a significantly small off-state current. For that reason, a potential of the gate electrode of the transistor 160 can be held for an extremely long period by turning off the transistor 162.

Note that there is no particular limitation on the transistor 160 and the transistor 166. In terms of increasing the speed of reading data, it is preferable to use a transistor with high switching speed such as a transistor formed using single crystal silicon, for example.

Furthermore, as illustrated in FIG. 1B, a capacitor 168 may be provided such that one electrode thereof is electrically connected to the gate electrode of the transistor 160 and the drain electrode (or the source electrode) of the transistor 162. The other electrode of the capacitor 168 is supplied with a predetermined potential. The predetermined potential is, for example, GND or the like. The other electrode of the capacitor 168 may be electrically connected to the first wiring. With the capacitor 168, more electric charge can be held at the gate electrode of the transistor 160, and data holding characteristics can be improved.

The semiconductor device in FIG. 1A utilizes a characteristic in which the potential of the gate electrode of the transistor 160 can be held, and can thus write, hold, and read data as follows.

First of all, writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode of the transistor 160. That is, predetermined charge is supplied to the gate electrode of the transistor 160 (writing). Here, one of two kinds of charges providing different potentials (hereinafter, a charge providing a low potential is referred to as charge Q_(L) and a charge providing a high potential is referred to as charge Q_(H)) is applied. Note that three or more kinds of charges providing different potentials may be applied to improve storage capacity. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge supplied to the gate electrode of the transistor 160 is held (holding).

Since the off-state current of the transistor 162 is significantly small, the charge of the gate electrode of the transistor 160 is held for a long time.

Next, reading of data will be described. By turning on the transistor 166 by supplying an appropriate potential (reading potential) to the fifth wiring while supplying a predetermined potential (a constant potential) to the first wiring, the potential of the second wiring varies depending on the amount of charge held at the gate electrode of the transistor 160. This is because, in the case where Q_(H) is supplied in writing, the transistor 160 is turned on, and in the case where Q_(L) is supplied, the transistor 160 is turned off. Therefore, the data held can be read by measuring the potential of the second wiring.

Note that in the case where memory cells are arrayed to be used, it is necessary that only data of a desired memory cell can be read. Therefore, as described above, the transistor 166 having the gate electrode connected to the fifth wiring is preferably provided between the second wiring and the transistor 160 where electric charge based on data is held at the gate electrode. In that state, the transistor 166 of a memory cell that is a target for reading is turned on by supplying an appropriate potential to the fifth wiring, and the transistor 166 of a memory cell that is not a target for reading is turned off by supplying a potential at which the transistor 166 is turned off, such as a ground potential, to the fifth wiring. Accordingly, the transistor 166 of the memory cell that is not a target for reading, which is electrically connected between the transistor 160 of the memory cell and the second wiring, can be selectively turned off regardless of electric charge held at the gate electrode of the transistor 160 of each memory cell. Thus, only the potential of the second wiring connected to the memory cell that is a target for reading can be read. Therefore, even when memory cells are arrayed, data of only a desired memory cell can be easily read.

Next, rewriting of data will be described. Rewriting of data is performed in a manner similar to that of the writing and holding of data. In other words, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring (a potential for new data) is supplied to the gate electrode of the transistor 160. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Accordingly, charge for new data is supplied to the gate electrode of the transistor 160.

In the semiconductor device according to an embodiment of the disclosed invention, data can be directly rewritten by another data writing operation as described above. Therefore, extraction of charge from a floating gate with the use of a high voltage which is necessary for a flash memory or the like is not needed, and thus a decrease in operation speed due to erasing operation can be suppressed. In other words, high-speed operation of the semiconductor device can be realized.

Note that the drain electrode (or the source electrode) of the transistor 162 is electrically connected to the gate electrode of the transistor 160 and therefore has a function similar to that of a floating gate of a floating gate transistor used for a nonvolatile memory element. A portion where the drain electrode (or the source electrode) of the transistor 162 and the gate electrode of the transistor 160 are electrically connected to each other is called a node FG in some cases. When the transistor 162 is turned off, the node FG can be regarded as being embedded in an insulator and thus charge is held at the node FG. The off-state current of the transistor 162 including an oxide semiconductor is smaller than or equal to 1/100000 of the off-state current of a transistor including a silicon semiconductor or the like; thus, loss of the charge accumulated in the node FG due to leakage in the transistor 162 is negligible. That is, with the transistor 162 including an oxide semiconductor, a nonvolatile memory device which can hold data without being supplied with power can be realized.

For example, when the off-state current of the transistor 162 at room temperature (25° C.) is 10 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, data can be held for 10⁴ seconds or longer. It is needless to say that the holding time depends on transistor characteristics and capacitance.

Further, the semiconductor device according to an embodiment of the disclosed invention does not have the problem of deterioration of a gate insulating film (tunnel insulating film), which is a problem of a conventional floating gate transistor. That is, the problem of deterioration of a gate insulating film due to injection of electrons into a floating gate or extraction of electrons from a floating gate, which is a conventional problem, can be solved. This means that there is no limit on the number of write cycles in principle. Furthermore, a high voltage needed for writing or erasing in a conventional floating gate transistor is not necessary.

Note that a multilevel technique can be employed in order to increase the storage capacity of the semiconductor device. For example, three or more levels of data are written to one memory cell, whereby the storage capacity can be increased as compared to the case where two-level (one-bit) data is written. The multilevel technique can be achieved by, for example, supplying charge Q to the gate electrode of the transistor 160, in addition to charge Q_(L) providing a low potential and charge Q_(H) providing a high potential. In that case, enough storage capacity can be ensured even in a circuit configuration with a relatively large scale.

Application Example 1

Next, a more specific circuit configuration to which the circuit illustrated in FIGS. 1A and 1B is applied and an operation thereof will be described with reference to FIGS. 2A and 2B, FIG. 3, FIG. 4, and FIGS. 5A to 5E.

FIGS. 2A and 2B are an example of a circuit diagram of a semiconductor device including (m×n) memory cells 170. The configuration of the memory cells 170 in FIGS. 2A and 2B is similar to that in FIG. 1B. That is, the first wiring in FIG. 1B corresponds to a source line SL in FIG. 2B; the second wiring in FIG. 1B, a bit line BL in FIGS. 2A and 2B; the third wiring in FIG. 1B, a signal line S in FIGS. 2A and 2B; the fourth wiring in FIG. 1B, a write word line WWL in FIGS. 2A and 2B; and the fifth wiring in FIG. 1B, a read word line RWL in FIGS. 2A and 2B. Note that the source line SL which is the first wiring in FIG. 1B is omitted in FIG. 2A.

The semiconductor device in FIG. 2A includes m (m is an integer greater than or equal to 2) write word lines WWL, m read word lines RWL, n (n is an integer greater than or equal to 2) bit lines BL, n signal lines S, a memory cell array having the memory cells 170 arranged in a matrix of m (rows) (in the vertical direction)×n (columns) (in the horizontal direction), a first driver circuit 190 connected to the n bit lines BL and the n signal lines S, and a second driver circuit 192 connected to the m write word lines WWL and the m read word lines RWL. Here, the memory cells 170 are connected in parallel between the source lines SL and the bit lines BL. A configuration in which the first driver circuit 190 and the second driver circuit 192 are connected by a wiring WRITE and a wiring READ is preferable.

In addition, address selection signal lines A_1 to A_m are connected to the second driver circuit 192. The address selection signal lines A_1 to A_m are wirings which transmit a signal for selecting a row address of memory cells.

FIG. 3 illustrates an example of specific configurations of the first driver circuit 190 and the second driver circuit 192 of the semiconductor device illustrated in FIGS. 2A and 2B. Note that the configurations of the first driver circuit 190 and the second driver circuit 192 are not limited to this example.

In FIG. 3, the first driver circuit 190 includes a control circuit 202 connected to an input terminal, a delay circuit 204 connected to the control circuit 202, a buffer circuit 206 connected to the delay circuit 204 and the signal line S, a reading circuit 208 connected to the bit line, and an output terminal through which a signal from the reading circuit 208 is output.

In addition, in FIG. 3, the second driver circuit 192 includes a decoder 210 connected to the address selection signal lines A_1 to A_m, a control circuit 212 connected to the decoder 210, a buffer circuit 216 connected to the control circuit 212 and the write word line WWL, and a buffer circuit 218 connected to the control circuit 212 and the read word line RWL. Here, a ground potential GND or a power supply potential VDD is output from the second driver circuit 192 to the write word line WWL and the read word line RWL.

Data writing, holding, and reading in the semiconductor device illustrated in FIGS. 2A and 2B are basically similar to those in the case of FIGS. 1A and 1B. A specific writing operation is as described below. Note that although the case where either a potential V1 (a potential lower than the power supply potential VDD by the threshold voltage of the transistor 162) or the ground potential GND is supplied to the node FG is described here as an example, the relationship among potentials supplied to the node FG is not limited to this example. Data that is held when the potential V1 is supplied to the node FG is referred to as data “1”, and data that is held when the ground potential GND (0 V) is supplied to the node FG is referred to as data “0”.

First, the memory cell 170 that is a target for writing is selected by setting the potentials of the read word line RWL and the write word line WWL, which are connected to the memory cell 170, to GND and VDD, respectively.

In order to write data “0” to the memory cell 170, GND is supplied to the signal line S, whereas in order to write data “1” to the memory cell 170, a potential is supplied to the signal line S in consideration of a potential decrease in the transistor 162 by its threshold voltage.

Data is held by setting the potential of the write word line WWL to the ground potential GND.

Because the ground potential GND is supplied to the write word line WWL, the transistor 162 is turned off regardless of whether data “1” or data “0” is written. Since the off-state current of the transistor 162 is significantly small, the charge of the gate electrode of the transistor 160 is held for a long time.

For data reading, the memory cell 170 that is a target for reading is selected by setting the potential of the read word line RWL to VDD and the potential of the write word line WWL to GND. Here, in each of the memory cells 170, the transistor 166 having the gate electrode connected to the read word line RWL is provided between the bit line BL and the transistor 160 where charge based on data is held at the gate electrode. Thus, the memory cell 170 that is the target for reading can be easily selected by setting the potential of the read word lines RWL connected to the memory cells that are not the target for reading to GND. In this manner, for data reading, the potential of the read word line RWL is simply set to the potential VDD or the ground potential GND. Therefore, there is no need to provide a potential conversion circuit in the semiconductor device illustrated in FIGS. 2A and 2B or to provide an additional external power source.

By raising the potential of the read word line RWL from GND to VDD, the potential of the gate electrode of the transistor 166 rises and becomes higher than or equal to the threshold voltage, and the transistor 166 is thus turned on.

By the above reading operation, if data “1” (the potential V1) is written in the memory cell 170, the transistor 160 is turned on and the potential of the bit line BL is decreased. If data “0” (the ground potential GND) is written, the transistor 160 is turned off and the potential of the bit line BL is maintained at the level at the beginning of reading or is raised.

FIG. 4 is an example of a timing chart for more detailed operations of the semiconductor device in FIGS. 2A and 2B. READ, A, and the like in the timing chart denote the lines to which the potentials in the timing chart are applied. Lines having a similar function are distinguished by “_(—)1”, “_(—)2”, and the like added to the end of their names. Note that for the sake of simplicity, the semiconductor device described here is an example where the memory cells 170 are arranged in 2 (rows)×2 (columns), but the disclosed invention is not limited to this example.

The timing chart in FIG. 4 shows the relationship among the potentials of the lines in the case where data “1” is written to all the memory cells (Writing 1), then the written data in all the memory cells is read out (Reading 1), then data “1” is written to the memory cells in the first row and the first column and in the second row and the second column while data “0” is written to the memory cells in the first row and the second column and in the second row and the first column (Writing 2), and then the written data in all the memory cells is read out (Reading 2).

In Writing 1, by setting WRITE at a high potential and READ at a low potential, the first driver circuit 190 and the second driver circuit 192 are put in such a condition that data can be written to the memory cells. The second driver circuit 192 outputs a row selection signal to WWL in accordance with the potentials of A_1 and A_2. Here, in the case where A_1 and A_2 are at a high potential, a target row is selected. In addition, WWL of a row selected is at a high potential, and RWL is at a low potential regardless of whether it is in a selected row or not.

In Writing 1, data “1” is written to all the memory cells; therefore, S_1 and S_2 are set at a high potential in accordance with the timing of row selection. Note that the period for inputting a signal to S_1 and S_2 is set as long as or longer than the period for inputting a signal to WWL. Alternatively, the input of a signal to S_1 and S_2 is delayed relative to the input of a signal to WWL. This is because there is a possibility that writing to memory cells may be insufficient if the period for inputting a signal to S_1 and S_2 is shorter than the period for inputting a signal to WWL. In order to make the period for inputting a signal to S_1 and S_2 longer than the period for inputting a signal to WWL, it is preferable that a delay circuit be connected to S_1 and S_2 and the input of a signal to S_1 and S_2 be delayed relative to the input of a signal to WWL. Alternatively, the input of a signal to S_1 and S_2 is delayed relative to the input of a signal to WWL by setting the channel length of transistors of buffer circuits connected to S_1 and S_2 larger than the channel length of a transistor of the buffer circuit connected to WWL. Alternatively, the input of a signal to S_1 and S_2 is delayed relative to the input of a signal to WWL by setting the channel width of transistors of the buffer circuits connected to S_1 and S_2 smaller than the channel width of a transistor of the buffer circuit connected to WWL. Note that the potentials of BL_1 and BL_2 are not an issue at the time of writing (the potentials may be either a high potential or a low potential).

In Reading 1, by setting READ at a high potential and WRITE at a low potential, the first driver circuit 190 and the second driver circuit 192 are put in such a condition that data can be read from the memory cells. The second driver circuit 192 outputs a row selection signal to RWL in accordance with the potentials of A_1 and A_1. Here, in the case where A_1 and A_2 are at a high potential, a target row is selected. In addition, RWL of a row selected is at a high potential, and WWL is at a low potential regardless of whether it is in a selected row or not.

By the above operation, potentials in accordance with data held by memory cells of a row selected are supplied to BL_1 and BL_2. Note that the potentials of S_1 and S_2 are not an issue at the time of reading.

The relationship among the potentials of the lines in Writing 2 is similar to that in Writing 1. Note that S_1 and S_2 are set at a high potential or a low potential in accordance with the timing of row selection in order to write data “1” to the memory cells in the first row and the first column and in the second row and the second column and in order to write data “0” to the memory cells in the first row and the second column and in the second row and the first column.

The relationship among the potentials of the lines in Reading 2 is similar to that in Reading 1. It is confirmed that the potential corresponding to the data held in the memory cells in the selected row is applied to the BL_1 and the BL_2.

Note that in order to delay the signal that is input to the signal line S relative to the signal that is input to the write word line WWL in the above writing operations, one of delay circuits illustrated in FIGS. 5A to 5E is preferably used as the delay circuit 204 in the first driver circuit 190 in FIG. 3 and connected to the signal line S. By connecting the delay circuit 204 and the signal line S, a change in potential of the signal line S can be delayed relative to a change in potential of the write word line WWL, and data can be sufficiently written to the memory cell 170.

As the delay circuit, a circuit in which an even number of inverter circuits are connected in series as illustrated in FIG. 5A can be used. Alternatively, the delay circuit may have a configuration in which a capacitor is added between an even number of inverter circuits connected in series as illustrated in FIG. 5B or a configuration in which a resistor is added between an even number of inverter circuits connected in series as illustrated in FIG. 5C. Still alternatively, the delay circuit may have a configuration in which a resistor and a capacitor are added between an even number of inverter circuits connected in series as illustrated in FIG. 5D. Still alternatively, the delay circuit may have a configuration in which an analog switch and a capacitor are added between an even number of inverter circuits connected in series as illustrated in FIG. 5E. Here, the analog switch is always on and functions as a resistor. In addition, since the analog switch has parasitic capacitance, the analog switch can further enhance the delaying effect.

Alternatively, in order to delay the signal that is input to the signal line S relative to the signal that is input to the write word line WWL in the above writing operations, the channel length of a transistor of the buffer circuit included in the first driver circuit 190 may be set larger than the channel length of a transistor of the buffer circuit included in the second driver circuit 192. Alternatively, the channel width of a transistor of the buffer circuit included in the first driver circuit 190 may be set smaller than the channel width of a transistor of the buffer circuit included in the second driver circuit 192. Also in that case, a change in potential of the signal line S can be delayed relative to a change in potential of the write word line WWL, and errors in writing data to the memory cell 170 can be reduced.

The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments.

Application Example 2

A circuit configuration which is different from the circuit configuration illustrated in FIGS. 2A and 2B will be described with reference to FIGS. 6A and 6B, FIG. 7, FIG. 8, and FIGS. 9A and 9B.

FIGS. 6A and 6B are an example of a circuit diagram of a semiconductor device including (m×n) memory cells 170. The configuration of the memory cells 170 in FIGS. 6A and 6B is similar to that in FIGS. 2A and 2B; therefore, the detailed description thereof is omitted.

The semiconductor device in FIG. 6A includes m (m is an integer greater than or equal to 2) write word lines WWL, m read word lines RWL, n (n is an integer greater than or equal to 2) bit lines BL, n signal lines S, a memory cell array having the memory cells 170 arranged in a matrix of m (rows) (in the vertical direction)×n (columns) (in the horizontal direction), a potential conversion circuit 180, a first driver circuit 190 connected to the n bit lines BL and the n signal lines S, and a second driver circuit 192 connected to the m write word lines WWL and the m read word lines RWL. Here, the potential conversion circuit 180 is connected to the second driver circuit 192 through a wiring VHL and outputs a potential (a high potential: VH) higher than a power supply potential to the second driver circuit 192. Note that the configuration in which a wiring WRITE is connected to the potential conversion circuit 180 is employed in this embodiment so as to convert potentials in accordance with the output of the first driver circuit 190. In addition, a configuration in which the first driver circuit 190 and the second driver circuit 192 are connected by the wiring WRITE and a wiring READ is preferable.

Furthermore, address selection signal lines A are connected to the second driver circuit 192. The address selection signal lines A are wirings which transmit a signal for selecting a row address of memory cells.

FIG. 7 illustrates an example of specific configurations of the first driver circuit 190 and the second driver circuit 192 of the semiconductor device illustrated in FIGS. 6A and 6B. Note that the configurations of the first driver circuit 190 and the second driver circuit 192 are not limited to this example.

In FIG. 7, the first driver circuit 190 includes a control circuit 202 connected to an input terminal, a delay circuit 204 connected to the control circuit 202, a buffer circuit 206 connected to the delay circuit 204 and the signal line S, a reading circuit 208 connected to the bit line, and an output terminal through which a signal from the reading circuit 208 is output. Note that the delay circuit 204 may be similar to the circuits illustrated in FIGS. 5A to 5E.

In addition, in FIG. 7, the second driver circuit 192 includes a decoder 210 connected to the address selection signal lines A, a control circuit 212 connected to the decoder 210, a step-up level shifter 214_H connected to the control circuit 212, a buffer circuit 216 connected to the step-up level shifter 214_H and the write word line WWL, and a buffer circuit 218 connected to the control circuit 212 and the read word line RWL. The step-up level shifter 214_H included in the second driver circuit 192 converts the amplitude of a signal with the use of the potential VH converted in the potential conversion circuit 180. Here, GND or VH is output to the write word line WWL, and GND or VDD is output to the read word line RWL.

Data writing, holding, and reading in the semiconductor device illustrated in FIGS. 6A and 6B are similar to those in the case of FIGS. 2A and 2B. Note that in the configuration illustrated in FIGS. 6A and 6B, the potential of the write word line WWL can be set at a potential (VH) higher than a power supply potential at the time of data writing. This can prevent the potential supplied to the node FG through the signal line S from being lowered at the transistor 162 by the threshold voltage. Therefore, a sufficiently high potential (e.g., VDD) can be supplied to the node FG, and data can be held for a longer time. In addition, data discrimination capability can be improved.

The potential conversion circuit 180 may be a step-up circuit or the like. An example of a four-stage step-up circuit is illustrated in FIG. 8. In FIG. 8, a power supply potential VDD is supplied to an input terminal (here, referring to a source terminal or a drain terminal which is connected to a gate terminal) of a first transistor 1300. An output terminal (here, referring to the source terminal or the drain terminal which is not connected to the gate terminal) of the first transistor 1300 is connected to an input terminal of a second transistor 1310 and one terminal of a first capacitor 1350. Similarly, an output terminal of the second transistor 1310 is connected to an input terminal of a third transistor 1320 and one terminal of a second capacitor 1360. Connections in the following stages are similar to the above, and therefore, detailed explanation is omitted. The connection can be represented as follows: an output terminal of an n-th transistor is connected to one terminal of an n-th capacitor (n: a natural number). In FIG. 8, an output terminal of a transistor of the last stage is connected to a transistor 1390 that is connected to a power source VDD; however, the disclosed invention is not limited to this configuration. For example, a configuration in which a capacitor connected to a ground potential GND is additionally provided may be employed. Note that in FIG. 8, an output of a fifth transistor 1340 is an output VH of the step-up circuit.

In addition, a clock signal CP_CLK is input to the other terminal of the second capacitor 1360 and the other terminal of a fourth capacitor 1380. A clock signal obtained by inverting the clock signal CP_CLK is input to the other terminal of the first capacitor 1350 and the other terminal of a third capacitor 1370. That is, the clock signal CP_CLK is input to the other terminal of a 2k-th capacitor and the inverted clock signal is input to the other terminal of a (2k−1)-th capacitor (k: a natural number). It is needless to say that the clock signal CP_CLK and the inverted clock signal can be interchanged.

When the clock signal CP_CLK is low, that is, when the inverted clock signal is high, the second capacitor 1360 and the fourth capacitor 1380 are charged. Potentials of a node N1 and a node N3 which are capacitively coupled with the inverted clock signal are raised by a predetermined voltage (a voltage corresponding to a difference between the high and low potentials of the clock signal CP_CLK). On the other hand, potentials of a node N2 and a node N4 which are capacitively coupled with the clock signal CP_CLK are dropped by a predetermined voltage.

Accordingly, charge is transferred through the second transistor 1310 and the fourth transistor 1330, and the potentials of the node N2 and the node N4 are raised to a predetermined value.

Next, when the clock signal CP_CLK becomes high and the inverted clock signal becomes low, the potentials of the node N2 and the node N4 are further raised. On the other hand, the potentials of the node N1 and the node N3 are dropped by a predetermined voltage.

Accordingly, charge is transferred through the first transistor 1300, the third transistor 1320 and the fifth transistor 1340, and as a result, the potentials of the node N1, the node N3 and the node N5 are raised to a predetermined potential. Thus, the potentials of the nodes satisfy V_(N5)=V_(N4(CP) _(—) _(CLK=High))>V_(N3(CP) _(—) _(CLK=Low))>V_(N2(CP) _(—) _(CLK=High))>V_(N1(CP) _(—) _(CLK=Low))>VDD, whereby step-up is performed. Note that the step-up circuit is not limited to a four-stage step-up circuit. The number of stages of the step-up circuit can be changed as appropriate. Note that n-channel transistors can be used as the first transistor 1300, the second transistor 1310, the third transistor 1320, the fourth transistor 1330 and the fifth transistor 1340 and that a p-channel transistor can be used as the transistor 1390.

Note that with the use of a transistor including an oxide semiconductor with favorable off-state current characteristics as a transistor included in the step-up circuit, the voltage of each node can be held for a longer time.

A signal whose amplitude is converted by the step-up level shifter included in the second driver circuit 192 with the use of a high potential converted in the potential conversion circuit 180 is output to the memory cells 170 through the write word line WWL. Furthermore, a configuration in which a signal whose amplitude is converted by a step-up level shifter included in the first driver circuit 190 with the use of a high potential converted in the potential conversion circuit 180 is output to the memory cells 170 through the signal line S may be employed.

FIGS. 9A and 9B each illustrate an example of a circuit diagram of a step-up level shifter. The level shifter illustrated in FIG. 9A has the following configuration. A source terminal of a first p-type transistor 1200 and a source terminal of a third p-type transistor 1230 are both electrically connected to a power source which supplies a potential VH. A drain terminal of the first p-type transistor 1200 is electrically connected to a source terminal of a second p-type transistor 1210, and a drain terminal of the third p-type transistor 1230 is electrically connected to a source terminal of a fourth p-type transistor 1240. A drain terminal of the second p-type transistor 1210 is electrically connected to a drain terminal of a first n-type transistor 1220 and a gate terminal of the third p-type transistor 1230, and a drain terminal of the fourth p-type transistor 1240 is electrically connected to a drain terminal of a second n-type transistor 1250 and a gate terminal of the first p-type transistor 1200. GND (=0 V) is supplied to both a source terminal of the first n-type transistor 1220 and a source terminal of the second n-type transistor 1250.

In FIG. 9A, an input signal (I) is input to a gate terminal of the second p-type transistor 1210 and a gate terminal of the first n-type transistor 1220, and an inverted signal (IB) of the input signal is input to a gate terminal of the fourth p-type transistor 1240 and a gate terminal of the second n-type transistor 1250. An output signal (O) is taken out from the drain terminal of the fourth p-type transistor 1240. In addition, an inverted signal (OB) of the output signal can be taken out from the drain terminal of the second p-type transistor 1210.

A basic operation of the level shifter illustrated in FIG. 9A will be described. When the input signal (I) is high, the first n-type transistor 1220 is turned on. Thus, the potential GND is input to the gate terminal of the third p-type transistor 1230 and the third p-type transistor 1230 is turned on. In addition, the inverted signal (OB) of the output signal is low, and the potential at this time is GND. On the other hand, at that time, the inverted signal (IB) of the input signal is low. Thus, the fourth p-type transistor 1240 is turned on and the second n-type transistor 1250 is turned off. At that time, both the third p-type transistor 1230 and the fourth p-type transistor 1240 are turned on. Thus, the output signal (O) is high, and at that time, the potential is VH.

When the potential of the input signal (I) is low, the operation can be understood in a manner similar to the above because the level shifter illustrated in FIG. 9A has a symmetric structure; the output signal (O) is low, and at that time, the potential is GND.

In this manner, the output signal (O) whose amplitude is converted with respect to the input signal can be obtained.

FIG. 9B illustrates an example of a circuit diagram of a step-up level shifter which is different from that in FIG. 9A. A configuration of the level shifter illustrated in FIG. 9B is as follows. A source terminal of a first p-type transistor 1260 and a source terminal of a second p-type transistor 1280 are both electrically connected to a power source which supplies a potential VH. A drain terminal of a first n-type transistor 1270 is electrically connected to a drain terminal of the first p-type transistor 1260 and a gate terminal of the second p-type transistor 1280, and a drain terminal of a second n-type transistor 1290 is electrically connected to a drain terminal of the second p-type transistor 1280 and a gate terminal of the first p-type transistor 1260. GND (=0 V) is supplied to both a source terminal of the first n-type transistor 1270 and a source terminal of the second n-type transistor 1290.

In FIG. 9B, an input signal (I) is input to a gate terminal of the first n-type transistor 1270, and an inverted signal (IB) of the input signal is input to a gate terminal of the second n-type transistor 1290. An output signal (O) is taken out from the drain terminal of the second n-type transistor 1290. In addition, an inverted signal (OB) of the output signal can be taken out from the drain terminal of the first n-type transistor 1270.

A basic operation of the level shifter illustrated in FIG. 9B will be described. When the input signal (I) is high, the first n-type transistor 1270 is turned on. Thus, the potential GND is input to the gate terminal of the second p-type transistor 1280 and the second p-type transistor 1280 is turned on. In addition, the inverted signal (OB) of the output signal is low, and the potential at this time is GND. On the other hand, at that time, the inverted signal (IB) of the input signal is low. Thus, the second n-type transistor 1290 is turned off. At that time, the second p-type transistor 1280 is turned on. Thus, the output signal (O) is high, and at that time, the potential is VH.

When the potential of the input signal (I) is low, the operation can be understood in a manner similar to the above because the level shifter illustrated in FIG. 9B has a symmetric structure; the output signal (O) is low, and at that time, the potential is GND.

In this manner, the output signal (O) whose amplitude is converted with respect to the input signal can be obtained.

<Reading Circuit>

Next, an example of the reading circuit 208 which can be used for the semiconductor devices illustrated in FIG. 3 and FIG. 7 and the like will be described with reference to FIGS. 10A to 10C.

FIG. 10A illustrates an outline of the reading circuit. The reading circuit includes a transistor and a sense amplifier circuit.

At the time of reading data, a terminal A is connected to a bit line BL to which a memory cell from which data is to be read is connected. Further, a bias potential Vbias is applied to a gate electrode of the transistor so that a potential of the terminal A is controlled.

The resistance of the memory cell 170 varies depending on stored data. Specifically, when the transistor 160 of the memory cell 170 selected is turned on, the memory cell 170 has a low resistance, whereas when the transistor 160 of the memory cell 170 selected is turned off, the memory cell 170 has a high resistance.

When the memory cell has a high resistance, the potential of the terminal A is higher than a reference potential Vref and the sense amplifier circuit outputs a potential (data “0”) corresponding to the potential of the terminal A. On the other hand, when the memory cell has a low resistance, the potential of the terminal A is lower than the reference potential Vref and the sense amplifier circuit outputs a potential (data “1”) corresponding to the potential of the terminal A.

In this manner, by using the reading circuit, data can be read from the memory cell. Note that the reading circuit of this embodiment is one example. Another known circuit may be used. The reading circuit may further include a precharge circuit. Instead of the reference potential Vref, a bit line connected to a reference memory cell may be connected to the sense amplifier circuit.

FIG. 10B illustrates a differential sense amplifier which is an example of the sense amplifier circuit. The differential sense amplifier has input terminals Vin(+) and Vin(−) and an output terminal Vout, and amplifies a difference between Vin(+) and Vin(−). If Vin(+)>Vin(−), the output from Vout is relatively high, whereas if Vin(+)<Vin(−), the output from Vout is relatively low.

FIG. 10C illustrates a latch sense amplifier which is an example of the sense amplifier circuit. The latch sense amplifier has input/output terminals V1 and V2 and input terminals for control signals Sp and Sn. First, the signal Sp is set high and the signal Sn is set low, and a power supply potential (Vdd) is interrupted. Then, potentials to be compared are supplied to V1 and V2. After that, the signal Sp is set low and the signal Sn is set high, and the power supply potential (Vdd) is supplied. If the potentials V1in and V2 in to be compared satisfy V1in>V2 in, the output from V1 is high and the output from V2 is low, whereas if the potentials satisfy V1in<V2 in, the output from V1 is low and the output from V2 is high. By utilizing such a relationship, the difference between V1in and V2 in can be amplified.

The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a configuration of a semiconductor device according to one embodiment of the disclosed invention and a manufacturing method of the semiconductor device will be described with reference to FIGS. 11A and 11B, FIGS. 12A to 12D, FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C, and FIG. 16.

FIG. 16 is a planar layout of a semiconductor device corresponding to the circuit illustrated in FIG. 2B. The transistor 160, the transistor 166, and the capacitor 168 are provided over a substrate. The transistor 160 includes a semiconductor layer 220, a first gate insulating layer, a gate electrode 110 a, a source or drain electrode 242 a which also functions as the source line SL, and a source or drain electrode 242 b. The transistor 166 includes the semiconductor layer 220, the first gate insulating layer, a gate electrode 110 b which also functions as the read word line RWL, the source or drain electrode 242 b, and a source or drain electrode 242 c which also functions as the bit line BL. The capacitor 168 includes a first electrode (a semiconductor layer 222 having conductivity), a second electrode (a conductive layer 224) which is connected to the source or drain electrode 242 a through a contact hole, and the first gate insulating layer which functions as a dielectric.

An insulating layer is provided over the transistor 160, the transistor 166, and the capacitor 168, and the transistor 162 is provided over the insulating layer. The transistor 162 includes a gate electrode 148 a which is electrically connected to a wiring 226 functioning as the write word line WWL through a conductive layer 240, an oxide semiconductor layer 144, a second gate insulating layer, a source electrode 142 a which is electrically connected to the gate electrode 110 a and the semiconductor layer 222 through a contact hole, and a drain electrode 142 b which is electrically connected to a wiring 156 functioning as the signal line S. An insulating layer is preferably provided over the transistor 162.

The semiconductor layer 220 and the semiconductor layer 222 having conductivity can be formed using a material similar to a semiconductor material used for a substrate 100 mentioned below. The gate electrode 110 a and the gate electrode 110 b can be formed using any of the materials mentioned below, and the conductive layer 224 and the wiring 226 can also be formed using a similar material. The gate electrode 148 a can be formed using any of the materials mentioned below. The oxide semiconductor layer 144 can be formed using any of the materials mentioned below. The source electrode 142 a and the drain electrode 142 b can be formed using any of the materials mentioned below, and the source or drain electrodes 242 a to 242 c and the conductive layer 240 can also be formed using a similar material.

A highly integrated planar layout of the semiconductor device illustrated in FIG. 16 will be described below.

<Cross-Sectional Structure and Planar Structure of Semiconductor Device>

FIGS. 11A and 11B illustrate an example of a highly integrated structure of the semiconductor device corresponding to the circuit illustrated in FIG. 2B. FIG. 11A is a cross-sectional view of the semiconductor device, and FIG. 11B is a plan view of the semiconductor device. Here, FIG. 11A corresponds to a cross section along line A1-A2 and line B1-B2 in FIG. 11B. The semiconductor device illustrated in FIGS. 11A and 11B includes, in a lower portion, the transistor 160 and the transistor 166 including a first semiconductor material, and in an upper portion, the transistor 162 including a second semiconductor material. Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold charge for a long time owing to its characteristics.

Although all of the above transistors are n-channel transistors in the following description, it is needless to say that p-channel transistors can be used. The technical nature of the disclosed invention is to use a semiconductor material with which off-state current can be sufficiently decreased, such as an oxide semiconductor, in the transistor 162 so that data can be held. Therefore, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to the structure described here. Although the transistor 160, the transistor 162, and the transistor 166 in the semiconductor device illustrated in FIGS. 11A and 11B have a top-gate structure, the structure of transistors used in the semiconductor device is not limited to the top-gate structure. For example, the transistor 162 may have a bottom-gate structure.

The transistor 160 in FIGS. 11A and 11B includes a channel formation region 116 a provided in a substrate 100 including a semiconductor material (such as silicon), an impurity region 120 a and an impurity region 120 b provided such that the channel formation region 116 a is sandwiched therebetween, a metal compound region 124 a and a metal compound region 124 b in contact with the impurity region 120 a and the impurity region 120 b, a gate insulating layer 108 a provided over the channel formation region 116 a, and a gate electrode 110 a provided over the gate insulating layer 108 a. Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may also be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode may be collectively referred to as a “source electrode”, and a drain region and a drain electrode may be collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region and the term “drain electrode” may include a drain region.

Like the transistor 160, the transistor 166 in FIGS. 11A and 11B also includes a channel formation region 116 b provided in the substrate 100 including a semiconductor material (such as silicon), the impurity region 120 b and an impurity region 120 c provided such that the channel formation region 116 b is sandwiched therebetween, the metal compound region 124 b and a metal compound region 124 c in contact with the impurity region 120 b and the impurity region 120 c, a gate insulating layer 108 b provided over the channel formation region 116 b, and a gate electrode 110 b provided over the gate insulating layer 108 b. When a conductive layer 142 c is provided over the gate electrode 110 b, the gate electrode 110 b can be prevented from being etched at the same time as forming a source electrode and a drain electrode of the transistor 162 by etching.

Further, an element isolation insulating layer 106 is formed over the substrate 100 so as to surround the transistor 160 and the transistor 166, and an insulating layer 130 is formed so as to cover the transistor 160 and the transistor 166. Note that in order to realize higher integration, the transistor 160 and the transistor 166 preferably have a structure without a sidewall insulating layer as illustrated in FIGS. 11A and 11B. On the other hand, in the case where characteristics of the transistor 160 and the transistor 166 have priority, a sidewall insulating layer may be provided on side surfaces of the gate electrode 110 a and the gate electrode 110 b, and the impurity regions 120 a to 120 c may include a region having a different impurity concentration.

The transistor 162 in FIGS. 11A and 11B includes a source electrode 142 a and a drain electrode 142 b provided over the insulating layer 130, an oxide semiconductor layer 144 electrically connected to the source electrode 142 a and the drain electrode 142 b, a gate insulating layer 146 covering the source electrode 142 a, the drain electrode 142 b, and the oxide semiconductor layer 144, and a gate electrode 148 a provided over the gate insulating layer 146 so as to overlap with the oxide semiconductor layer 144.

Here, the oxide semiconductor layer 144 is preferably an oxide semiconductor layer which is purified by sufficiently removing an impurity such as hydrogen therefrom or by sufficiently supplying oxygen thereto. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less, for example. Note that the above hydrogen concentration of the oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). The carrier concentration of the oxide semiconductor layer 144, in which hydrogen is reduced to a sufficiently low concentration so that the oxide semiconductor layer is purified and in which defect states in an energy gap due to oxygen deficiency are reduced by sufficiently supplying oxygen as described above, is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. For example, the off-state current (per unit channel width (1 μm), here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less, more preferably 1 zA or less, still more preferably 100 yA or less. It is preferable that the off-state current of the transistor 162 be as small as possible, but the lower limit of the off-state current of the transistor 162 is estimated at 10⁻³⁰ A. In this manner, by using an i-type (intrinsic) or substantially i-type oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

Although the oxide semiconductor layer 144 processed in an island shape is used in the transistor 162 of FIGS. 11A and 11B in order to suppress leakage among elements due to miniaturization, an oxide semiconductor layer not processed in an island shape may be used. When an oxide semiconductor layer is not processed in an island shape, the oxide semiconductor layer 144 can be prevented from being contaminated by etching during processing.

The capacitor 168 in FIGS. 11A and 11B includes the source electrode 142 a, the oxide semiconductor layer 144, the gate insulating layer 146, and an electrode 148 b. In other words, the source electrode 142 a functions as one electrode of the capacitor 168, and the electrode 148 b functions as the other electrode of the capacitor 168.

Note that in the capacitor 168 of FIGS. 11A and 11B, the oxide semiconductor layer 144 and the gate insulating layer 146 are stacked, whereby insulation between the source electrode 142 a and the electrode 148 b can be sufficiently secured. Here, a ground potential or the like is preferably supplied to the electrode 148 b. It is needless to say that the capacitor 168 without including the oxide semiconductor layer 144 may be employed in order to secure sufficient capacitance. Furthermore, in the case where no capacitor is needed, a structure in which the capacitor 168 is not provided is also possible.

Note that in the transistor 162 and the capacitor 168, the source electrode 142 a and the drain electrode 142 b preferably have tapered end portions. The source electrode 142 a and the drain electrode 142 b preferably have tapered end portions because the coverage thereof with the oxide semiconductor layer 144 can be improved and disconnection thereof can be prevented. Here, the taper angle is 30° to 60°, for example. Note that the “taper angle” means an angle formed by the side surface and the bottom surface of a layer having a tapered shape (for example, the source electrode 142 a) when observed from a direction perpendicular to a cross section thereof (a plane perpendicular to the substrate surface).

In the semiconductor device illustrated in FIGS. 11A and 11B, the transistor 162 and the capacitor 168 are provided so as to overlap with the transistor 160. By employing such a planar layout, higher integration can be realized. For example, given that the minimum feature size is F, the area occupied by a memory cell can be approximately 15 F² to 25 F².

An insulating layer 150 is provided over the transistor 162 and the capacitor 168, and an insulating layer 152 is provided over the insulating layer 150. Then, an electrode 154 is provided in an opening formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 152, and the like, and a wiring 156 is formed over the insulating layer 152 so as to be connected to the electrode 154. Although the drain electrode 142 b and the wiring 156 are connected by the electrode 154 in FIGS. 11A and 11B, the disclosed invention is not limited to this structure. For example, the wiring 156 may be in direct contact with the drain electrode 142 b.

<Method for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the semiconductor device will be described. First, a method for manufacturing the transistor 160 and the transistor 166 in the lower portion will be described below with reference to FIGS. 12A to 12D and FIGS. 13A to 13C; then, a method for manufacturing the transistor 162 in the upper portion and the capacitor 168 will be described with reference to FIGS. 14A to 14C and FIGS. 15A to 15C.

<Method for Manufacturing Transistors in Lower Portion>

First, the substrate 100 including a semiconductor material is prepared (see FIG. 12A). A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like, a compound semiconductor substrate of silicon germanium or the like, an SOI substrate, or the like can be used as the substrate 100 including a semiconductor material. Here, an example of the case where a single crystal silicon substrate is used as the substrate 100 including a semiconductor material is described. Note that the term “SOI substrate” generally means a substrate where a silicon semiconductor layer is provided over an insulating surface. In this specification and the like, the term “SOI substrate” also means a substrate where a semiconductor layer including a material other than silicon is provided over an insulating surface. That is, a semiconductor layer included in the “SOI substrate” is not limited to a silicon semiconductor layer. Moreover, the SOI substrate can be a substrate having a structure where a semiconductor layer is provided over an insulating substrate such as a glass substrate with an insulating layer interposed therebetween.

It is preferable that a single crystal semiconductor substrate of silicon or the like be particularly used as the substrate 100 including a semiconductor material because the speed of reading operation of the semiconductor device can be increased.

A protective layer 102 serving as a mask for forming an element isolation insulating layer is formed over the substrate 100 (see FIG. 12A). As the protective layer 102, an insulating layer formed using a material such as silicon oxide, silicon nitride, silicon oxynitride, or the like can be used, for example. Note that before or after this step, an impurity element imparting n-type conductivity or an impurity element imparting p-type conductivity may be added to the substrate 100 in order to control the threshold voltage of the transistor. When the semiconductor material included in the substrate 100 is silicon, phosphorus, arsenic, or the like can be used as the impurity imparting n-type conductivity. Boron, aluminum, gallium, or the like can be used as the impurity imparting p-type conductivity.

Next, part of the substrate 100 in a region not covered with the protective layer 102 (i.e., in an exposed region) is removed by etching using the protective layer 102 as a mask. Thus, a semiconductor region 104 isolated from other semiconductor regions is formed (see FIG. 12B). As the etching, dry etching is preferably performed, but wet etching may be performed. An etching gas or an etchant can be selected as appropriate depending on a material to be etched.

Then, an insulating layer is formed so as to cover the semiconductor region 104, and the insulating layer in a region overlapping with the semiconductor region 104 is selectively removed; thus, the element isolation insulating layer 106 is formed (see FIG. 12C). The insulating layer is formed using silicon oxide, silicon nitride, silicon oxynitride, or the like. As a method for removing the insulating layer, any of etching treatment, polishing treatment such as chemical mechanical polishing (CMP), and the like can be employed. Note that the protective layer 102 is removed after the formation of the semiconductor region 104 or after the formation of the element isolation insulating layer 106.

Next, an insulating layer is formed over a surface of the semiconductor region 104, and a layer including a conductive material is formed over the insulating layer.

The insulating layer is processed into a gate insulating layer later and can be formed by, for example, heat treatment (thermal oxidation treatment, thermal nitridation treatment, or the like) of the surface of the semiconductor region 104. Instead of heat treatment, high-density plasma treatment may be employed. The high-density plasma treatment can be performed using, for example, a mixed gas of any of a rare gas such as He, Ar, Kr, or Xe, oxygen, nitrogen oxide, ammonia, nitrogen, hydrogen, and the like. It is needless to say that the insulating layer may be formed by a CVD method, a sputtering method, or the like. The insulating layer preferably has a single-layer structure or a stacked-layer structure with a film including silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added, or the like. The insulating layer can have a thickness of 1 nm to 100 nm, preferably, 10 nm to 50 nm, for example.

The layer including a conductive material can be formed using a metal material such as aluminum, copper, titanium, tantalum, or tungsten. The layer including a conductive material may be formed using a semiconductor material such as polycrystalline silicon. There is no particular limitation on the method for forming the layer including a conductive material, and a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, or a spin coating method can be employed. Note that this embodiment shows an example of the case where the layer including a conductive material is formed using a metal material.

After that, the insulating layer and the layer including a conductive material are selectively etched; thus, the gate insulating layer 108 a, the gate insulating layer 108 b, the gate electrode 110 a, and the gate electrode 110 b are formed (see FIG. 12C).

Next, phosphorus (P), arsenic (As), or the like is added to the semiconductor region 104, whereby the channel formation region 116 a, the channel formation region 116 b, and the impurity regions 120 a to 120 c are formed (see FIG. 12D). Note that phosphorus or arsenic is added here in order to form an n-type transistor; an impurity element such as boron (B) or aluminum (Al) may be added in the case of forming a p-type transistor. Here, the concentration of the impurity added can be set as appropriate; the concentration is preferably set high when a semiconductor element is highly miniaturized.

Note that a sidewall insulating layer may be formed around the gate electrode 110 a and the gate electrode 110 b, and impurity regions to which the impurity element is added at a different concentration may be formed.

Next, a metal layer 122 is formed so as to cover the gate electrode 110 a, the gate electrode 110 b, the impurity regions 120 a to 120 c, and the like (see FIG. 13A). The metal layer 122 can be formed by a variety of film formation methods such as a vacuum evaporation method, a sputtering method, and a spin coating method. The metal layer 122 is preferably formed using a metal material which forms a low-resistance metal compound by reacting with the semiconductor material included in the semiconductor region 104. Examples of such metal materials are titanium, tantalum, tungsten, nickel, cobalt, platinum, and the like.

Next, heat treatment is performed so that the metal layer 122 reacts with the semiconductor material. Thus, the metal compound regions 124 a to 124 c that are in contact with the impurity regions 120 a to 120 c are formed (see FIG. 13A). Note that when the gate electrode 110 a and the gate electrode 110 b are formed using polycrystalline silicon or the like, a metal compound region is also formed in portions of the gate electrode 110 a and the gate electrode 110 b which are in contact with the metal layer 122.

As the heat treatment, irradiation with a flash lamp can be employed, for example. Although it is needless to say that another heat treatment method may be used, a method by which heat treatment can be achieved in an extremely short time is preferably used in order to improve the controllability of chemical reaction for formation of the metal compound. Note that the metal compound regions are formed by reaction of the metal material and the semiconductor material and have sufficiently high conductivity. The formation of the metal compound regions can properly reduce the electric resistance and improve element characteristics. Note that the metal layer 122 is removed after the metal compound regions 124 a to 124 c are formed.

Next, the insulating layer 130 is formed so as to cover the components formed in the above steps (see FIG. 13B). The insulating layer 130 can be formed using an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, or aluminum oxide. It is particularly preferable to use a low dielectric constant (low-k) material for the insulating layer 130 because capacitance due to overlap of electrodes or wirings can be sufficiently reduced. Note that a porous insulating layer with such a material may be employed as the insulating layer 130. The porous insulating layer has a lower dielectric constant than an insulating layer with high density and thus makes it possible to further reduce capacitance due to electrodes or wirings. Alternatively, the insulating layer 130 can be formed using an organic insulating material such as polyimide or acrylic. Note that the insulating layer 130 may have a single-layer structure or a stacked-layer structure including two or more layers.

Through the above steps, the transistor 160 and the transistor 166 are formed with the use of the substrate 100 including a semiconductor material (see FIG. 13B). A feature of the transistor 160 and the transistor 166 is that they can operate at high speed. With the use of these transistors as a reading transistor and a transistor for selecting a memory cell at the time of reading, data can be read at high speed.

After that, as treatment performed before the transistor 162 and the capacitor 168 are formed, CMP treatment of the insulating layer 130 is performed so that upper surfaces of the gate electrode 110 a and the gate electrode 110 b are exposed (see FIG. 13C). As treatment for exposing the upper surfaces of the gate electrode 110 a and the gate electrode 110 b, etching treatment or the like can also be employed instead of CMP treatment; in order to improve characteristics of the transistor 162, a surface of the insulating layer 130 are preferably made as flat as possible.

Note that before or after each of the above steps, a step of forming an electrode, a wiring, a semiconductor layer, an insulating layer, or the like may be further performed. For example, when the wiring has a multi-layer structure of a stacked-layer structure including insulating layers and conductive layers, a highly integrated semiconductor device can be realized.

<Method for Manufacturing Transistor in Upper Portion>

Next, a conductive layer is formed over the gate electrode 110 a, the gate electrode 110 b, the insulating layer 130, or the like, and the source electrode 142 a, the drain electrode 142 b, and the conductive layer 142 c are formed by selectively etching the conductive layer (see FIG. 14A).

The conductive layer can be formed by a PVD method such as a sputtering method, or a CVD method such as a plasma CVD method. As a material of the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloy including any of these elements as a component, or the like can be used. A material including one of manganese, magnesium, zirconium, beryllium, neodymium, or scandium or a combination of a plurality of these elements may be used.

The conductive layer may have a single-layer structure or a stacked-layer structure including two or more layers. For example, the conductive layer may have a single-layer structure of a titanium film or a titanium nitride film, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, or the like. Note that the conductive layer having a single-layer structure of a titanium film or a titanium nitride film has an advantage in that it can be easily processed into the source electrode 142 a and the drain electrode 142 b having a tapered shape.

The conductive layer may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which is abbreviated to ITO in some cases), an indium oxide-zinc oxide alloy (In₂O₃—ZnO), or any of these metal oxide materials including silicon or silicon oxide can be used.

The conductive layer is preferably etched such that the source electrode 142 a and the drain electrode 142 b are formed to have tapered end portions. Here, the taper angle is preferably 30° to 60°, for example. When the source electrode 142 a and the drain electrode 142 b are formed by etching so as to have tapered end portions, coverage of the source electrode 142 a and the drain electrode 142 b with the gate insulating layer 146 which is formed later can be improved and disconnection of the gate insulating layer 146 can be prevented.

The channel length (L) of the transistor in the upper portion is determined by a distance between lower edge portions of the source electrode 142 a and the drain electrode 142 b. Note that for light exposure for forming a mask in the case of manufacturing a transistor with a channel length (L) of less than 25 nm, light exposure is preferably performed with extreme ultraviolet light whose wavelength is several nanometers to several tens of nanometers, which is extremely short. The resolution of light exposure with extreme ultraviolet rays is high and the depth of focus is large. For these reasons, the channel length (L) of the transistor to be formed later can be set in the range of 10 nm to 1000 nm (1 μm), and the circuit can operate at higher speed. In addition, power consumption of the semiconductor device can be reduced by miniaturization.

When the conductive layer 142 c is provided over the gate electrode 110 b, the gate electrode 110 b can be prevented from being etched at the same time as etching the conductive layer.

Note that an insulating layer functioning as a base may be provided over the insulating layer 130. The insulating layer can be formed by a PVD method, a CVD method, or the like.

Next, the oxide semiconductor layer 144 is formed by forming an oxide semiconductor layer so as to cover the source electrode 142 a and the drain electrode 142 b and then by selectively etching the oxide semiconductor layer (see FIG. 14B).

The oxide semiconductor layer can be formed using a four-component metal oxide such as In—Sn—Ga—Zn—O-based oxide semiconductor, a three-component metal oxide such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor, a two-component metal oxide such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxide semiconductor, a single-component metal oxide such as an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor, or the like. In addition, any of the above oxide semiconductors may contain an element other than In, Ga, Sn, and Zn, for example, SiO₂.

In particular, an In—Ga—Zn—O-based oxide semiconductor material has sufficiently high resistance when there is no electric field and thus off-state current can be sufficiently reduced. In addition, also having high field-effect mobility, the In—Ga—Zn—O-based oxide semiconductor material is suitable for a semiconductor material used in a semiconductor device.

As a typical example of the In—Ga—Zn—O-based oxide semiconductor material, an oxide semiconductor material represented by InGaO₃(ZnO)_(m) (m>0) is given. Using M instead of Ga, there is an oxide semiconductor material represented by InMO₃(ZnO), (m>0). Here, M denotes one or more metal elements selected from gallium (Ga), aluminum (Al), iron (Fe), nickel (Ni), manganese (Mn), cobalt (Co), or the like. For example, M may be Ga, Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga and Co, or the like. Note that the above-described compositions are derived from the crystal structures that the oxide semiconductor material can have and are mere examples.

As a target used for forming the oxide semiconductor layer by a sputtering method, a target having a composition ratio of In:Ga:Zn=1:x:y (x is greater than or equal to 0 and y is greater than or equal to 0.5 and less than or equal to 5) is preferably used. For example, a target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] (x=1, y=1) or the like can be used. Furthermore, a target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] (x=1, y=0.5), a target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] (x=1, y=2), or a target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:0:2 [molar ratio] (x=0, y=1) can also be used.

In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

In this embodiment, the oxide semiconductor layer is formed by a sputtering method with the use of an In—Ga—Zn—O-based metal oxide target.

The relative density of the metal oxide in the metal oxide target is 80% or more, preferably 95% or more, and more preferably 99.9% or more. The use of the metal oxide target with high relative density makes it possible to form an oxide semiconductor layer having a dense structure.

The atmosphere in which the oxide semiconductor layer is formed is preferably a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas (typically, argon) and oxygen. Specifically, it is preferable to use a high-purity gas atmosphere, for example, from which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to a concentration of 1 ppm or less (preferably, 10 ppb or less).

In forming the oxide semiconductor layer, for example, an object to be processed is held in a treatment chamber that is maintained under reduced pressure, and the object to be processed is heated to a temperature higher than or equal to 100° C. and lower than 550° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Alternatively, the temperature of an object to be processed in forming the oxide semiconductor layer may be room temperature (25° C.±10° C.). Then, moisture in the treatment chamber is removed, a sputtering gas from which hydrogen, water, or the like is removed is introduced, and the above-described target is used; thus, the oxide semiconductor layer is formed. By forming the oxide semiconductor layer while heating the object to be processed, an impurity in the oxide semiconductor layer can be reduced. Moreover, damage due to sputtering can be reduced. In order to remove the moisture in the treatment chamber, it is preferable to use an entrapment vacuum pump. For example, a cryopump, an ion pump, a titanium sublimation pump, or the like can be used. A turbomolecular pump provided with a cold trap may be used. Since hydrogen, water, or the like can be removed from the treatment chamber evacuated with a cryopump or the like, the concentration of an impurity in the oxide semiconductor layer can be reduced.

For example, conditions for forming the oxide semiconductor layer can be set as follows: the distance between the object to be processed and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power is 0.5 kW, and the atmosphere is an oxygen (100% oxygen) atmosphere, an argon (100% argon) atmosphere, or a mixed atmosphere of oxygen and argon. Note that a pulsed direct current (DC) power source is preferably used because dust (such as powder substances generated in film formation) can be reduced and the film thickness can be uniform. The thickness of the oxide semiconductor layer is set in the range of 1 nm to 50 nm, preferably 1 nm to 30 nm, more preferably 1 nm to 10 nm. The use of the oxide semiconductor layer of such a thickness makes it possible to suppress a short channel effect which is caused by miniaturization. Note that the appropriate thickness of the oxide semiconductor layer differs depending on the oxide semiconductor material to be used, the intended use of the semiconductor device, or the like; therefore, the thickness can be determined as appropriate in accordance with the material, the intended use, or the like.

Note that before the oxide semiconductor layer is formed by a sputtering method, reverse sputtering in which plasma is generated with an argon gas introduced is preferably performed so that a material attached to a formation surface (e.g., a surface of the insulating layer 130) is removed. Here, the reverse sputtering is a method in which ions collide with a surface to be processed so that the surface is modified, in contrast to normal sputtering in which ions collide with a sputtering target. An example of a method for making ions collide with a surface to be processed is a method in which high-frequency voltage is applied to the surface side in an argon atmosphere so that plasma is generated near the object to be processed. Note that an atmosphere of nitrogen, helium, oxygen, or the like may be used instead of an argon atmosphere.

After that, heat treatment (first heat treatment) is preferably performed on the oxide semiconductor layer. Through the first heat treatment, excess hydrogen (including water or a hydroxyl group) in the oxide semiconductor layer can be removed, the structure of the oxide semiconductor layer can be ordered, and defect states in an energy gap can be reduced. For example, the temperature of the first heat treatment can be set higher than or equal to 300° C. and lower than 550° C., or higher than or equal to 400° C. and lower than or equal to 500° C.

For example, after an object to be processed is introduced into an electric furnace including a resistance heater or the like, the heat treatment can be performed at 450° C. for one hour in a nitrogen atmosphere. The oxide semiconductor layer is not exposed to the air during the heat treatment so that entry of water or hydrogen can be prevented.

The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object to be processed by thermal radiation or thermal conduction from a medium such as a heated gas. For example, a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas that does not react with an object to be processed by heat treatment, for example, nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, GRTA treatment may be performed as follows. The object to be processed is put in a heated inert gas atmosphere, heated for several minutes, and taken out of the inert gas atmosphere. The GRTA treatment enables high-temperature heat treatment in a short time. Moreover, the GRTA treatment can be employed even when the temperature exceeds the upper temperature limit of the object to be processed. Note that the inert gas may be switched to a gas including oxygen during the treatment. This is because defect states in an energy gap caused by oxygen vacancies can be reduced by performing the first heat treatment in an atmosphere including oxygen.

Note that as the inert gas atmosphere, an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is set to 6N (99.9999%) or more, preferably 7N (99.99999%) or more (i.e., the impurity concentration is 1 ppm or less, preferably 0.1 ppm or less).

In any case, a transistor with extremely excellent characteristics can be obtained with the use of the oxide semiconductor layer which is an i-type (intrinsic) or substantially i-type oxide semiconductor layer obtained by reducing an impurity through the first heat treatment.

The above heat treatment (the first heat treatment) can also be referred to as dehydration treatment, dehydrogenation treatment, or the like because it has the effect of removing hydrogen, water, or the like. The dehydration treatment or the dehydrogenation treatment can be performed after the oxide semiconductor layer is formed, after the gate insulating layer is formed, or after a gate electrode is formed. Such dehydration treatment or dehydrogenation treatment may be performed once or plural times.

The etching of the oxide semiconductor layer may be performed either before the heat treatment or after the heat treatment. Dry etching is preferably used in terms of element miniaturization, but wet etching may be used. An etching gas or an etchant can be selected as appropriate depending on a material to be etched. Note that in the case where leakage in an element or the like does not cause a problem, the oxide semiconductor layer does not necessarily need to be processed in an island shape.

Next, the gate insulating layer 146 is formed in contact with the oxide semiconductor layer 144. Then, over the gate insulating layer 146, the gate electrode 148 a is formed in a region overlapping with the oxide semiconductor layer 144, and the electrode 148 b is formed in a region overlapping with the source electrode 142 a (see FIG. 14C).

The gate insulating layer 146 can be formed by a CVD method, a sputtering method, or the like. The gate insulating layer 146 is preferably formed so as to contain silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide, yttrium oxide, gallium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added, or the like. The gate insulating layer 146 may have a single-layer structure or a stacked-layer structure. There is no particular limitation on the thickness of the gate insulating layer 146; the thickness is preferably small in order to ensure the operation of the transistor when the semiconductor device is miniaturized. For example, in the case of using silicon oxide, the thickness can be 1 nm to 100 nm, preferably 10 nm to 50 nm.

However, when the gate insulating layer is thin, gate leakage due to a tunneling effect or the like becomes a problem. In order to solve the problem of gate leakage, the gate insulating layer 146 may be formed using a high dielectric constant (high-k) material such as hafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added. The use of a high-k material for the gate insulating layer 146 makes it possible to increase the thickness in order to suppress gate leakage as well as ensuring electrical properties. Note that a stacked-layer structure of a film including a high-k material and a film including any of silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, and the like may also be employed.

After the gate insulating layer 146 is formed, second heat treatment is preferably performed in an inert gas atmosphere or an oxygen atmosphere. The temperature of the heat treatment is set in the range of 200° C. to 450° C., preferably 250° C. to 350° C. For example, the heat treatment may be performed at 250° C. for one hour in a nitrogen atmosphere. By the second heat treatment, variation in electrical characteristics of the transistor can be reduced. In the case where the gate insulating layer 146 contains oxygen, oxygen can be supplied to the oxide semiconductor layer 144 and oxygen vacancies in the oxide semiconductor layer 144 can be filled; thus, the oxide semiconductor layer which is i-type (intrinsic) or substantially i-type can also be formed.

Note that the second heat treatment is performed in this embodiment after the gate insulating layer 146 is formed; there is no limitation on the timing of the second heat treatment. For example, the second heat treatment may be performed after the gate electrode is formed. Alternatively, the first heat treatment and the second heat treatment may be performed in succession, or the first heat treatment may double as the second heat treatment, or the second heat treatment may double as the first heat treatment.

By performing at least one of the first heat treatment and the second heat treatment as described above, the oxide semiconductor layer 144 can be purified so as to contain impurities other than main components as little as possible.

The gate electrode 148 a and the electrode 148 b can be formed by forming a conductive layer over the gate insulating layer 146 and then by selectively etching the conductive layer. The conductive layer to be the gate electrode 148 a and the electrode 148 b can be formed by a PVD method such as a sputtering method, or a CVD method such as a plasma CVD method. The details are similar to those of the source electrode 142 a or the like; thus, the description thereof can be referred to.

Next, the insulating layer 150 and the insulating layer 152 are formed over the gate insulating layer 146, the gate electrode 148 a, and the electrode 148 b (see FIG. 15A). The insulating layer 150 and the insulating layer 152 can be formed by a PVD method, a CVD method, or the like. The insulating layer 150 and the insulating layer 152 can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, or aluminum oxide.

Note that the insulating layer 150 and the insulating layer 152 are preferably formed using a low dielectric constant material or a low dielectric constant structure (such as a porous structure). This is because when the insulating layer 150 and the insulating layer 152 have a low dielectric constant, capacitance generated between wirings, electrodes, or the like can be reduced and operation at higher speed can be achieved.

Note that although a stacked-layer structure of the insulating layer 150 and the insulating layer 152 is used in this embodiment, an embodiment of the disclosed invention is not limited to this example. A single-layer structure or a stacked-layer structure including three or more layers can also be used. Alternatively, a structure in which the insulating layers are not provided is also possible.

Note that the insulating layer 152 is desirably formed so as to have a flat surface. This is because when the insulating layer 152 has a flat surface, an electrode, a wiring, or the like can be favorably formed over the insulating layer 152 even in the case where the semiconductor device or the like is miniaturized. Note that the insulating layer 152 can be planarized using a method such as chemical mechanical polishing (CMP).

Next, an opening reaching the drain electrode 142 b is formed in the gate insulating layer 146, the insulating layer 150, and the insulating layer 152 (see FIG. 15B). The opening is formed by selective etching with a mask or the like.

After that, the electrode 154 is formed in the opening, and the wiring 156 in contact with the electrode 154 is formed over the insulating layer 152 (see FIG. 15C).

The electrode 154 can be formed in such a manner, for example, that a conductive layer is formed in a region including the opening by a PVD method, a CVD method, or the like and then part of the conductive layer is removed by etching, CMP, or the like.

Specifically, it is possible to employ a method, for example, in which a thin titanium film is formed in a region including the opening by a PVD method and a thin titanium nitride film is formed by a CVD method, and then, a tungsten film is formed so as to be embedded in the opening. Here, the titanium film formed by a PVD method functions to reduce an oxide film (e.g., a natural oxide film) formed on a surface where the titanium film is formed, and to decrease the contact resistance with a lower electrode or the like (here, the drain electrode 142 b). The titanium nitride film formed after the formation of the titanium film has a barrier function for suppressing diffusion of the conductive material. A copper film may be formed by a plating method after the formation of a barrier film of titanium, titanium nitride, or the like.

Note that in the case where the electrode 154 is formed by removing part of the conductive layer, the process is preferably performed so that the surface is planarized. For example, when a thin titanium film or a thin titanium nitride film is formed in a region including the opening and then a tungsten film is formed so as to be embedded in the opening, excess tungsten, titanium, titanium nitride, or the like is removed and the planarity of the surface can be improved by subsequent CMP treatment. The surface including the electrode 154 is planarized in this manner, so that an electrode, a wiring, an insulating layer, a semiconductor layer, or the like can be favorably formed in a later step.

The wiring 156 is formed by forming a conductive layer by a PVD method such as a sputtering method, or a CVD method such as a plasma CVD method, and then by patterning the conductive layer. As a material of the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloy including any of these elements as a component, or the like can be used. A material including one of manganese, magnesium, zirconium, beryllium, neodymium, and scandium or a combination of a plurality of these elements may be used. The details are similar to those of the source electrode 142 a and the like.

Through the above steps, the transistor 162 including the oxide semiconductor layer 144 which is purified and the capacitor 168 are completed (see FIG. 15C).

In the transistor 162 described in this embodiment, the oxide semiconductor layer 144 is purified and thus contains hydrogen at a concentration of 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less. In addition, the carrier density of the oxide semiconductor layer 144 is, for example, less than 1×10¹²/cm³, preferably less than 1.45×10¹⁰/cm³, which is sufficiently lower than the carrier density of a general silicon wafer (approximately 1×10¹⁴/cm³). In addition, the off-state current of the transistor 162 is sufficiently small. For example, the off-state current (per unit channel width (1 μm), here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less, more preferably 1 zA or less, still more preferably 100 yA or less. It is preferable that the off-state current of the transistor 162 be as small as possible, but the lower limit of the off-state current of the transistor 162 is estimated at 10⁻³⁰ A.

In this manner, by using the oxide semiconductor layer 144 which is purified and is intrinsic, it becomes easy to sufficiently reduce the off-state current of the transistor. With the use of such a transistor, a semiconductor device in which stored data can be held for an extremely long time can be provided.

The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, the cases where the semiconductor device described in any of the above embodiments is applied to electronic devices will be described with reference to FIGS. 17A to 17F. The cases where the above-described semiconductor device is applied to electronic devices such as a computer, a mobile phone set (also referred to as a mobile phone or a mobile phone device), a portable information terminal (including a portable game machine, an audio reproducing device, and the like), a digital camera, a digital video camera, electronic paper, a television set (also referred to as a television or a television receiver), and the like are described in this embodiment.

FIG. 17A illustrates a notebook personal computer, which includes a housing 701, a housing 702, a display portion 703, a keyboard 704, and the like. The semiconductor device described in any of the above embodiments is provided in at least one of the housings 701 and 702. Thus, a notebook personal computer with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

FIG. 17B illustrates a portable information terminal (PDA). A main body 711 is provided with a display portion 713, an external interface 715, operation buttons 714, and the like. Further, a stylus 712 for operation of the portable information terminal, or the like is provided. The semiconductor device described in any of the above embodiments is provided in the main body 711. Thus, a portable information terminal with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

FIG. 17C illustrates an electronic book 720 incorporating electronic paper, which includes two housings, a housing 721 and a housing 723. The housing 721 and the housing 723 include a display portion 725 and a display portion 727, respectively. The housing 721 is connected to the housing 723 by a hinge 737, so that the electronic book can be opened and closed using the hinge 737 as an axis. In addition, the housing 721 is provided with a power switch 731, operation keys 733, a speaker 735, and the like. At least one of the housings 721 and 723 is provided with the semiconductor device described in any of the above embodiments. Thus, an electronic book with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

FIG. 17D illustrates a mobile phone set, which includes two housings, a housing 740 and a housing 741. Moreover, the housings 740 and 741 in a state where they are developed as illustrated in FIG. 17D can be slid so that one is lapped over the other. Therefore, the size of the mobile phone set can be reduced, which makes the mobile phone set suitable for being carried around. The housing 741 includes a display panel 742, a speaker 743, a microphone 744, a pointing device 746, a camera lens 747, an external connection terminal 748, and the like. The housing 740 includes a solar cell 749 for charging the mobile phone set, an external memory slot 750, and the like. Here, the display panel 742 has a touch panel function. A plurality of operation keys 745 displayed as images is indicated by dashed lines in FIG. 17D. An antenna is incorporated in the housing 741. The semiconductor device described in any of the above embodiments is provided in at least one of the housings 740 and 741. Thus, a mobile phone set with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

FIG. 17E illustrates a digital camera, which includes a main body 761, a display portion 767, an eyepiece 763, an operation switch 764, a display portion 765, a battery 766, and the like. The semiconductor device described in any of the above embodiments is provided in the main body 761. Thus, a digital camera with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

FIG. 17F is a television set 770, which includes a housing 771, a display portion 773, a stand 775, and the like. The television set 770 can be operated with a switch included in the housing 771 or with a remote controller 780. The semiconductor device described in any of the above embodiments is mounted in the housing 771 and the remote controller 780. Thus, a television set with sufficiently low power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.

As described above, the electronic devices described in this embodiment each include the semiconductor device according to any of the above embodiments. Therefore, electronic devices with low power consumption can be realized.

Example 1

In this example, results of measuring the off-state current of a transistor including a purified oxide semiconductor will be described.

First, a transistor with a channel width W of 1 m, which is sufficiently wide, was prepared in consideration of a very small off-state current of a transistor including a purified oxide semiconductor, and the off-state current was measured. FIG. 18 shows the results of measuring the off-state current of the transistor with a channel width W of 1 m. In FIG. 18, the horizontal axis shows a gate voltage VG and the vertical axis shows a drain current ID. In the case where the drain voltage VD is +1 V or +10 V and the gate voltage VG is within the range of −5 V to −20 V, the off-state current of the transistor is found to be smaller than or equal to 1×10⁻¹² A which is the detection limit. In addition, the off-state current (per unit channel width (1 μm), here) of the transistor is found to be smaller than or equal to 1 aA/μm (1×10⁻¹⁸ A/μm).

Next, the results of more accurately measuring the off-state current of the transistor including a purified oxide semiconductor will be described. As described above, the off-state current of the transistor including a purified oxide semiconductor is found to be smaller than or equal to 1×10⁻¹² A which is the detection limit of measurement equipment. Here, the results of measuring more accurate off-state current (a value smaller than or equal to the detection limit of measurement equipment in the above measurement) with the use of an element for characteristic evaluation will be described.

First, the element for characteristic evaluation which is used in a method for measuring current will be described with reference to FIG. 19.

In the element for characteristic evaluation in FIG. 19, three measurement systems 800 are connected in parallel. The measurement systems 800 each include a capacitor 802, a transistor 804, a transistor 805, a transistor 806, and a transistor 808. As the transistor 804, the transistor 805, the transistor 806, and the transistor 808, transistors including a purified oxide semiconductor were employed.

In the measurement system 800, one of a source terminal and a drain terminal of the transistor 804, one of terminals of the capacitor 802, and one of a source terminal and a drain terminal of the transistor 805 are electrically connected to a power source (for supplying V2). The other of the source terminal and the drain terminal of the transistor 804, one of a source terminal and a drain terminal of the transistor 808, the other of the terminals of the capacitor 802, and a gate terminal of the transistor 805 are electrically connected to one another. The other of the source terminal and the drain terminal of the transistor 808, one of a source terminal and a drain terminal of the transistor 806, and a gate terminal of the transistor 806 are electrically connected to a power source (for supplying V1). The other of the source terminal and the drain terminal of the transistor 805 and the other of the source terminal and the drain terminal of the transistor 806 are electrically connected to each other and electrically connected to an output terminal.

Note that a potential Vext_b2 for controlling whether to turn on or off the transistor 804 is supplied to the gate terminal of the transistor 804, and a potential Vext_b1 for controlling whether to turn on or off the transistor 808 is supplied to the gate terminal of the transistor 808. A potential Vout is output from the output terminal.

Next, a method for measuring current with the use of the element for characteristic evaluation will be described.

First, an initial period in which a potential difference is generated to measure the off-state current will be briefly described. In the initial period, the potential Vext_b1 for turning on the transistor 808 is input to the gate terminal of the transistor 808. Accordingly, a potential V1 is supplied to a node A that is electrically connected to the other of the source terminal and the drain terminal of the transistor 804 (that is, the node electrically connected to one of the source terminal and the drain terminal of the transistor 808, the other of the terminals of the capacitor 802, and the gate terminal of the transistor 805). Here, the potential V1 is, for example, a high potential. In addition, the transistor 804 is turned off.

After that, the potential Vext_b1 for turning off the transistor 808 is input to the gate terminal of the transistor 808, so that the transistor 808 is turned off. After the transistor 808 is turned off, the potential V1 is set to a low potential. Still, the transistor 804 is turned off. The potential V2 is equal to the potential V1. Thus, the initial period is completed. When the initial period is completed, a potential difference is generated between the node A and one of the source terminal and the drain terminal of the transistor 804. In addition, a potential difference is generated between the node A and the other of the source terminal and the drain terminal of the transistor 808. Accordingly, a small amount of electric charge flows through the transistor 804 and the transistor 808. That is, the off-state current is generated.

Next, a measurement period of the off-state current will be briefly described. In the measurement period, the potential (that is, V2) of one of the source terminal and the drain terminal of the transistor 804 and the potential (that is, V1) of the other of the source terminal and the drain terminal of the transistor 808 are fixed to a low potential. On the other hand, the potential of the node A is not fixed (the node A is in a floating state) in the measurement period. Accordingly, charge flows through the transistor 804, and the amount of charge held at the node A changes over time. The potential of the node A changes depending on the change in the amount of charge held at the node A. That is, the output potential Vout of the output terminal also changes.

FIG. 20 shows details (a timing chart) of the relationship among potentials in the initial period in which the potential difference is generated and those in the subsequent measurement period.

In the initial period, first, the potential Vext_b2 is set to a potential (a high potential) at which the transistor 804 is turned on. Thus, the potential of the node A becomes V2, that is, a low potential (VSS). Note that it is not essential to supply a low potential (VSS) to the node A. After that, the potential Vext_b2 is set to a potential (a low potential) at which the transistor 804 is turned off, so that the transistor 804 is turned off. Next, the potential Vext_b1 is set to a potential (a high potential) at which the transistor 808 is turned on. Accordingly, the potential of the node A becomes V1, that is, a high potential (VDD). Then, the potential Vext_b1 is set to a potential at which the transistor 808 is turned off, which places the node A in a floating state and finishes the initial period.

In the measurement period after the initial period, the potential V1 and the potential V2 are set such that charge flows to the node A or charge flows out of the node A. Here, the potential V1 and the potential V2 are set to a low potential (VSS). Note that at the time when the output potential Vout is measured, it is necessary to operate an output circuit and thus temporarily set V1 to a high potential (VDD) in some cases. Note that the period in which V1 is set to a high potential (VDD) is made short to such a degree that the measurement is not influenced.

When the potential difference is generated and the measurement period is started as described above, the amount of charge held at the node A changes over time, which causes the potential of the node A to change. This means that the potential of the gate terminal of the transistor 805 changes; thus, the output potential Vout of the output terminal also changes over time.

A method for calculating the off-state current on the basis of the obtained output potential Vout is described below.

The relationship between a potential V_(A) of the node A and the output potential Vout is obtained before calculation of the off-state current. With this relationship, the potential V_(A) of the node A can be obtained using the output potential Vout. In accordance with the above relationship, the potential V_(A) of the node A can be expressed as a function of the output potential Vout by the following equation. V _(A) =F(Vout)  [Formula 1]

Charge Q_(A) of the node A can be expressed by the following equation with the use of the potential V_(A) of the node A, capacitance C_(A) connected to the node A, and a constant (const). Here, the capacitance C_(A) connected to the node A is the sum of the capacitance of the capacitor 802 and other capacitance. Q _(A) =C _(A) V _(A)+const  [Formula 2]

Current I_(A) of the node A is a time derivative of charge which flows to the node A (or charge which flows out of the node A), and is thus expressed by the following equation.

$\begin{matrix} {I_{A} = {\frac{\Delta\; Q_{A}}{\Delta\; t} = \frac{{C_{A} \cdot \Delta}\;{F({Vout})}}{\Delta\; t}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \end{matrix}$

In this manner, the current I_(A) of the node A can be obtained from the capacitance C_(A) connected to the node A and the output potential Vout of the output terminal.

In accordance with the above method, it is possible to measure leakage current (off-state current) which flows between a source and a drain of a transistor in an off state.

In this example, the transistor 804, the transistor 805, the transistor 806, and the transistor 808 were manufactured using a purified oxide semiconductor with a channel length L of 10 μn and a channel width W of 50 μm. In addition, in the measurement systems 800 which are arranged in parallel, the capacitances of the capacitors 802 were 100 fF, 1 pF, and 3 pF.

Note that VDD was 5 V and VSS was 0 V in the measurement of this example. In the measurement period, Vout was measured while the potential V1 was basically set to VSS and changed to VDD for 100 msec at intervals of 10 sec to 300 sec. In addition, Δt used in calculation of current I which flows through the element was approximately 30000 sec.

FIG. 21 shows the relationship between the elapsed time Time in the above current measurement and the output potential Vout. It can be confirmed from FIG. 21 that the potential changes as time elapses.

FIG. 22 shows the off-state current at room temperature (25° C.) which is calculated in the above current measurement. Note that FIG. 22 shows the relationship between the source-drain voltage V and the off-state current I. It is found from FIG. 22 that the off-state current is about 40 zA/μm under the condition where the source-drain voltage is 4 V. It is also found that the off-state current is smaller than or equal to 10 zA/μm under the condition where the source-drain voltage is 3.1 V. Note that 1 zA represents 10⁻²¹ A.

Furthermore, FIG. 23 shows the off-state current in an environment at a temperature of 85° C., which is calculated in the above current measurement. FIG. 23 shows the relationship between the source-drain voltage V and the off-state current I in an environment at a temperature of 85° C. It is found from FIG. 23 that the off-state current is smaller than or equal to 100 zA/μm under the condition where the source-drain voltage is 3.1 V.

As described above, it is confirmed from this example that the off-state current of a transistor including a purified oxide semiconductor is sufficiently small.

Example 2

In this example, detailed results of measuring the off-state current Ioff of a transistor including an intrinsic oxide semiconductor will be described. Note that Sentaurus Device (TCAD software produced by Synopsys, Inc.) was used for calculation. The SRH model and the Auger recombination model were used as carrier recombination models.

The structure of a transistor used for the calculation is as illustrated in FIG. 24. A transistor 562 illustrated in FIG. 24 includes an insulating layer 543 (material: silicon oxide), a source electrode 542 a and a drain electrode 542 b (material: titanium nitride), an oxide semiconductor layer 544 (material: In—Ga—Zn—O-based oxide semiconductor, thickness: 10 nm) in contact with part of an upper surface of the insulating layer 543, an upper surface of the source electrode 542 a, and an upper surface of the drain electrode 542 b, a gate insulating layer 546 (material: hafnium oxide, thickness: 10 nm) covering the oxide semiconductor layer 544, and a gate electrode 548 (material: tungsten) over the gate insulating layer 546. Note that the calculation was based on the assumption that the oxide semiconductor was i-type and the donor density N_(d) was equal to the intrinsic carrier density n_(i).

As the channel length L, two conditions of 50 nm and 500 nm were adopted. The voltage Vds between the source electrode 542 a and the drain electrode 542 b was set to 1 V.

Parameters used for the calculation are given below.

1. In—Ga—Zn—O-based oxide semiconductor (material of the oxide semiconductor layer)

Band gap Eg: 3.15 eV, electron affinity χ: 4.3 eV, relative permittivity: 15, electron mobility: 10 cm²/Vs

2. Titanium nitride (material of the source electrode and the drain electrode)

Work function φ_(M): 3.9 eV

3. Hafnium oxide (material of the gate insulating layer)

Relative permittivity: 15

4. Tungsten (material of the gate electrode)

Work function φ_(M): 4.9 eV

FIG. 25 shows calculation results. In FIG. 25, the horizontal axis represents the gate voltage VG (V), and the vertical axis represents the drain current ID (A/μm). In FIG. 25, a thicker line represents the results of calculation in the case where the channel length L=500 nm, and a thinner line represents the results of calculation in the case where the channel length L=50 nm.

It can be found from the calculation results in FIG. 25 that the lower limit of the off-state current is estimated at 10⁻²⁹ to 10⁻³⁰ (A/μm) either when the channel length L is 50 nm or when 500 nm.

In this example, the off-state current of a transistor including an intrinsic oxide semiconductor is calculated using the transistor having a top-gate structure illustrated in FIG. 24. Note that substantially the same off-state current can be obtained with a transistor having a different structure such as a bottom-gate structure.

This application is based on Japanese Patent Application serial no. 2010-095196 filed with Japan Patent Office on Apr. 16, 2010, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A semiconductor device comprising: a write word line; a read word line; a bit line; a source line; a signal line; a memory cell comprising: a first transistor comprising a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region; a second transistor comprising a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region; and a third transistor comprising a third gate electrode, a third source electrode, a third drain electrode, and a third channel formation region; a first driver circuit comprising a delay circuit electrically connected to the signal line; and a second driver circuit, wherein the first channel formation region comprises a semiconductor material different from a semiconductor material of the second channel formation region, wherein the first gate electrode and the second drain electrode are electrically connected to each other, wherein the first drain electrode and the third source electrode are electrically connected to each other, wherein the source line is electrically connected to the first source electrode, wherein the first driver circuit is electrically connected to the third drain electrode through the bit line and electrically connected to the second source electrode through the delay circuit and the signal line, and wherein the second driver circuit is electrically connected to the third gate electrode through the read word line and electrically connected to the second gate electrode through the write word line.
 2. The semiconductor device according to claim 1, further comprising a capacitor comprising a first electrode electrically connected to the first gate electrode and a second electrode electrically connected to the source line.
 3. The semiconductor device according to claim 1, further comprising a plurality of memory cells, wherein the memory cell and the plurality of memory cells are connected in parallel between the bit line and the source line.
 4. The semiconductor device according to claim 1, wherein the second channel formation region of the second transistor comprises an oxide semiconductor.
 5. The semiconductor device according to claim 1, wherein the third channel formation region comprises a same semiconductor material as the first channel formation region.
 6. A semiconductor device comprising: a write word line; a read word line; a bit line; a source line; a signal line; a memory cell comprising: a first transistor comprising a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region; a second transistor comprising a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region; and a third transistor comprising a third gate electrode, a third source electrode, a third drain electrode, and a third channel formation region; a first driver circuit comprising a delay circuit electrically connected to the signal line; a second driver circuit; and a potential conversion circuit, wherein the first channel formation region comprises a semiconductor material different from a semiconductor material of the second channel formation region, wherein the first gate electrode and the second drain electrode are electrically connected to each other, wherein the first drain electrode and the third source electrode are electrically connected to each other, wherein the source line is electrically connected to the first source electrode, wherein the first driver circuit is electrically connected to the third drain electrode through the bit line and electrically connected to the second source electrode through the delay circuit and the signal line, wherein the second driver circuit is electrically connected to the third gate electrode through the read word line and electrically connected to the second gate electrode through the write word line, and wherein the potential conversion circuit is configured to output a potential higher than a potential inputted into the potential conversion circuit to the second driver circuit.
 7. The semiconductor device according to claim 6, wherein the second driver circuit comprises a step-up level shifter electrically connected to the potential conversion circuit and the write word line.
 8. The semiconductor device according to claim 6, further comprising a capacitor comprising a first electrode electrically connected to the first gate electrode and a second electrode electrically connected to the source line.
 9. The semiconductor device according to claim 6, further comprising a plurality of memory cells, wherein the memory cell and the plurality of memory cells are connected in parallel between the bit line and the source line.
 10. The semiconductor device according to claim 6, wherein the second channel formation region of the second transistor comprises an oxide semiconductor.
 11. The semiconductor device according to claim 6, wherein the third channel formation region comprises a same semiconductor material as the first channel formation region. 